1. Field
Aspects of the present invention relate to a mask for sequential lateral solidification (SLS) and a SLS apparatus having the same, and more particularly, to a mask for SLS, capable of preventing an overlapping region and a diagonal stain based on a crystallization pattern of an active layer.
2. Description of the Related Art
In an active matrix (AM) type organic light emitting display device, each pixel includes a pixel driving circuit. The pixel driving circuit includes a thin-film transistor (TFT) using silicon. Amorphous silicon (a-Si) or polycrystalline silicon (poly-Si) is used as the silicon constituting the TFT.
In the a-Si TFT used in the pixel driving circuit, a semiconductor active layer has a source, a drain, and a channel and is formed of a-Si. Thus, the a-Si TFT exhibits relatively low electron mobility below 1 cm2/Vs. Therefore, the recent trend is to replace the a-Si TFT with a poly-Si TFT. As compared to the a-Si TFT, the poly-Si TFT exhibits relatively high electron mobility and excellent stability with respect to light irradiation. Therefore, it is highly appropriate to use a poly-Si TFT as an active layer in a driving and/or switching TFT of an active matrix (AM) organic light emitting display device.
Such a poly-Si may be fabricated by using various methods. The methods may be generally categorized into methods of directly depositing the poly-Si and methods of depositing a-Si and crystallizing the deposited a-Si.
Examples of the methods of directly depositing the poly-Si include chemical vapor deposition (CVD), photo CVD, hydrogen radical (HR) CVD, electron cyclotron resonance (ECR) CVD, plasma enhanced (PE) CVD, low pressure (LP) CVD, or the like. Examples of the methods of depositing the a-Si and crystallizing the deposited a-Si include solid phase crystallization (SPC), excimer laser crystallization (ELC), metal induced crystallization (MIC), metal induced lateral crystallization (MILC), sequential lateral solidification (SLS), or the like.
The SPC method is impractical because it is necessary to maintain a high temperature above 600° C. for an extended period of time. Although the ELC method has an advantage in terms of low-temperature crystallization, a laser beam is expanded by using optics, and thus uniformity is relatively low. Meanwhile, the MIC method may reduce the temperature of crystallization by depositing a metal thin-film on an a-Si surface and crystallizing the a-Si by using the metal thin-film as a crystallization catalyst. However, since an a-Si layer is polluted with a metal, a TFT formed of the a-Si layer according to the MIC method exhibits deteriorated characteristics, and crystals are relatively small and have non-uniform growth patterns.
The SLS is a method using the fact that grains of silicon grow in a direction perpendicular to a border surface between liquid-state silicon and solid-state silicon. The a-Si is crystallized by partially melting the a-Si by irradiating a laser beam through a particular portion of the a-Si using a mask and growing crystals from the border between the melted portion and portion not melted in the direction toward the melted portion. The SLS is being focused on as a method of fabricating a low temperature poly-Si.
FIG. 1 shows SLS equipment 32 for performing SLS. The SLS equipment 32 includes a laser emitting device 36, a condensing lens 40, a mask 38, and a reduction lens 42. The laser emitting device 36 emits a laser beam 34. The condenser lens 40 condenses the laser beam 34 emitted by the laser emitting device 36. The mask 38 selectively irradiates the laser beam 34 onto a substrate 44. The reduction lens 42 is located above or below the mask 38 and reduces the laser beam 34, which passed through the mask 38, to scale.
The laser beam emitting device 36 emits an unprocessed laser beam from a light source, adjusts an energy level of the laser beam via an attenuator (not shown), and irradiates the laser beam 34 via the condenser lens 40.
An x-y stage 46 holds the substrate 44 having an a-Si thin-film deposited thereon. The x-y stage 46 is located at a location corresponding to the mask 38. At this point, to crystallize the entire substrate 44, the region being crystallized is expanded through fine relocations of the x-y stage 46.
In the configuration stated above, the mask 38 includes transmitting regions A for transmitting the laser beam 34 and blocking regions B for blocking the laser beam 34. The width of each of the blocking regions B (the distance between the transmitting regions A) determines the length of lateral growth of crystal grains.
A method of crystallizing a-Si by using the conventional SLS equipment will be described below. Generally, a crystalline silicon is formed by forming a buffer layer (not shown), which is an insulation film, on the substrate 44, forming an a-Si layer on the top surface of the buffer layer, and using the a-Si layer. The a-Si layer is generally deposited on the substrate 44 via the CVD method, where the a-Si layer contains a large amount of hydrogen. Since the hydrogen characteristically escapes from the thin-film due to heat, it is necessary to perform primary thermal processing on the amorphous preceding film for dehydrogenation. The reason for this is that the surface of a crystalline thin-film becomes significantly uneven in the case where hydrogen is not removed in advance, and thus electric characteristics thereof are significantly deteriorated.
However, by using a crystallization method using a laser beam, the entire region of a substrate 44 cannot be crystallized simultaneously. The reason for this is that the beam width of a laser beam 34 and the size of a mask 38 are limited. Therefore, crystallization is completed by aligning a single mask 38 a plurality of times and repeating a crystallization process every time the mask 38 is aligned. At this point, if a crystallized region corresponding to a reduced area of the single mask 38 is defined as one block, the one block is also crystallized by laser beam 34 irradiations performed a plurality of times.
FIG. 2 is a plan view of a mask 60 used in silicon crystallization. The mask 60 is configured to obtain a 2-shot effect (completion of crystallizing one block with two irradiations) by performing a crystallization process with a single scan in the x-axis direction. As shown in FIG. 2, the mask 60 includes the transmitting regions A and the blocking region B, where the transmitting regions A are arranged in the form of stripes extending in a horizontal direction. At this point, the transmitting regions A are formed in the upper and lower portions of the mask 60 at predetermined intervals apart from each other, such that one of the transmitting regions A is located adjacent to the region between the transmitting regions A formed in the upper and lower portions of the mask 60.
A length d of the region between the transmitting regions A is smaller than a length L of the transmitting region A. The three transmitting regions A shown in FIG. 2 are formed to each have the same length L. In this case, when a laser beam is irradiated to the mask 60 from the upper portion of the mask 60, crystal grains are laterally grown from two opposite interfaces of the a-Si layer in the melted region. Growth of each of the laterally grown crystal grains is stopped as grain boundaries collide with each other, where no core generation region exists between the laterally grown crystal grains. Accordingly, the 2-shot effect may be obtained by a single scan in the x-axis direction.
FIGS. 3A, 3B, 3C, and 3D are plan views for showing a crystallization method using the structure of a mask used in general methods of forming poly-Si thin-films. As shown in FIG. 3A, a laser beam is irradiated onto a-Si formed on a substrate 70 while using the general mask 60 having a transmissive pattern A and a non-transmissive pattern B, and the poly-Si is formed as the a-Si is melted and solidified.
Then, as shown in FIG. 3B, when the mask 70 is shifted by a predetermined distance in the x-axis direction and the laser beam is irradiated again, poly-Si in a crystallized region at which the a-Si and a transmissive region overlap each other is melted and crystallized again as shown in FIG. 3C. In the same manner, the crystallization process is performed on poly-Si in regions where the a-Si and the transmissive regions overlap by being melted and solidified through repeated scans and laser beam irradiations.
As described above, after completing crystallization in the x-axis direction by successively crystallizing the a-Si below a mask 60 by moving the mask 60 in the x-axis direction and irradiating the laser beam to the upper portion of the mask 60, the mask 60 is relocated by a predetermined distance in the y-axis direction and is moved in the x-axis direction again to crystallize a-Si.
However, in this case, for a-Si to be continuously formed in the y-axis direction, it is necessary to perform crystallization after the mask 60 is relocated in the y-axis direction such that a portion Y-OL of the transmissive region A of the mask overlaps a portion of already crystallized a-Si. In other words, the overlapping region Y-OL, where a portion of an already crystallized region in the upper portion of the a-Si and a portion of a region to be crystallized in the lower portion of the a-Si overlap each other, is inevitably formed, and as such, the overlapping region Y-OL is an uneven region deteriorating overall crystallization quality.